IP Multimedia (IPMM) is an example of a service which provides a dynamic combination of voice, video, messaging, data, etc, within the same session. By growing the numbers of basic applications and the media which it is possible to combine, the number of services offered to the end users will grow, and the inter-personal communication experience will be enriched. This will lead to a new generation of personalized, rich multimedia communication services, e.g. peer-to-peer multimedia communication, IPTV etc. These services can be based on the IP Multimedia Subsystem (IMS) architecture, which is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks (3GPP TS 22.228, TS 23.228, TS 24.229, TS 29.228, TS 29.229, TS 29.328. and TS 29.329). The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). The Session Description Protocol (SDP), carried by SIP signaling, is used to describe and negotiate the media components of the session. Other multimedia applications which can be used for media transmission and control include Real-time Transport Protocol and Real-time Transport Control Protocol (RTP/RTCP), Message Session Relay Protocol (MSRP), and Hyper Text Transfer Protocol (HTTP). 3GPP2 and TISPAN describe support to access IMS infrastructure and services from other access networks than General Packet Radio Service (GPRS), such as CDMA2000. (Code Division Multiple Access 2000) and fixed line.
Several roles of Session Initiation Protocol servers or proxies, collectively called Call Session Control Function CSCF, are used to process Session Initiation Protocol SIP signalling packets in the IMS.
A Proxy-CSCF (P-CSCF) is a SIP proxy that is the first point of contact for a user terminal. It can be located either in the visited network or in the home network, when the visited network is not yet IMS compliant. The user terminal discovers its P-CSCF with either Domain Host Configuration Protocol (DHCP), or by signalling related to activation of PDP Context (Packet Data Protocol Context), as in GPRS, wherein the P-CSCF is assigned to the terminal. It is assigned to an terminal during SIP registration. It is situated on the path of signalling messages, and can inspect every message. It authenticates the user and establishes an IPsec security association with the user terminal. The Serving CSCF (S-CSCF) is a SIP proxy which provides services to the user that the user is subscribed to; and the Interrogating CSCF (I-CSCF) is a SIP proxy whose role is to identify the correct S-CSCF and to forward to that S-CSCF a request received from a SIP terminal via a P-CSCF.
A mobile user terminal with dual mode card is capable to access and operate under both WLAN and for example UMTS or GSM frequency bands and modulation schemes. For getting access to the packet core network Interworking WLAN (I-WLAN) technology has been introduced by 3GPP (see for instance Technical Specification TS 23.234). I-WLAN consists of two main parts:                Direct IP access:        3GPP IP access with data through a Packet Data Gateway PDG.It is possible to configure the PDG in different ways. One solution is a two node implementation comprising a Tunnel Termination Gateway TTG and a Gateway GPRS Support Node GGSN. IKEv2. signalling is used for establishing the IPsec tunnel between the user terminal and the TTG.        
A 3GPP user terminal with IMS client accessing packet core network via a traditional Radio Access Network RAN (e.g. GSM EDGE Radio Access Network, GERAN, or Universal Terrestrial Radio Access Network, UTRAN) and the Packet Switched (PS) domain will receive the P-CSCF address when activating the PDP context. This information is included in the Protocol Configuration Options (PCO) Information Element (IE) in the Activate PDP Context Accept message that the terminal receives from SGSN. SGSN receives the P-CSCF address information from the GGSN.
An IMS client using I-WLAN 3GPP IP Access needs to also obtain the relevant initiation information, in this case the P-CSCF address, to be able to access the IMS infrastructure and services. However, there is no specified way to forward the P-CSCF address to the terminal once it is received in the TTG from the GGSN. It is an advantage that the terminals receive the same IMS server information when using the different access networks.